Impression material compositions comprising alginates and divalent or polyvalent metal salts such as calcium sulfate, zinc oxide, etc. have been known to be used in taking impression as alginate impression materials. These alginate impression materials have advantages that they are good in precision of impression, can reproduce fine parts and are easy of impressing operation; accordingly, they have been widely used as e.g. impression material compositions for dental use. In general, impression material compositions in two forms of a powder and a paste, containing inorganic fillers, are used as required In the impression material compositions in both forms, the alginate and the gelling agent are reacted in the presence of water and gelled to form an elastic hardened product. In order to obtain precise impression, it is necessary that they are kneaded sufficiently and uniformly and incorporation of air is minimized. To this end, it is advisable that viscosity of a paste in kneading the impression material composition is as low as possible. When the kneaded paste is put on a tray and pressed against a final mold, more than fixed viscosity is needed. Otherwise, the paste is poured down, the tray can hardly be kept in a given position and a gap is formed between the mold and the impression material, making it difficult to provide precise impression. Moreover, a step of kneading the impression material composition with water to form a paste and a step of taking impression are usually finished only within several minutes. In practical use, the above opposite properties are required to be imparted to one impression material composition.
The conventional impression material compositions hardly meet all of the above required properties, and they are actually used by sacrificing either viscosity for the paste or viscosity in taking impression.